The present invention relates to a process for preparing methyl formate, to an apparatus for carrying out this process and to methyl formate which has been prepared by this process and is to be used, in particular, for preparing formic acid.
Methyl formate is used in industry predominantly for the preparation of formic acid and for this purpose is at present produced on a large scale by reaction of methanol with carbon monoxide in the presence of alkali metal alkoxide. However, the industrial implementation of this process suffers from problems which make the process inconvenient, unreliable and energy-consuming and thus considerably increase its costs.
To seek to perfect the process for industrial-scale use, the technology for carrying out the process has developed in essentially two directions: one group of processes employs a very high CO pressure with the aim of obtaining very high concentrations of methyl formate in the output from the reactor, while another group of processes makes use of a relatively low CO pressure with the aim of obtaining a relatively low concentration of methyl formate in the reaction mixture so as to avoid formation of salt deposits in the reactors, on cooling surfaces and in valves to make trouble-free operation possible over a relatively long time. DE-C-926 785 discloses a high-pressure process in which the reaction mixture is circulated and flows alternately through a reactor and a cooler. Fresh methanol and sodium methoxide dissolved therein as catalyst are fed to the top of the reactor, and the carbon monoxide or the gas mixture comprising carbon monoxide is injected under a pressure of 300 bar at the bottom of the reactor operating at from 80 to 130° C. Part of the circulating reaction mixture is continuously discharged via a pressure chamber. The process is carried out using a low catalyst concentration of not more than 0.25% by weight of sodium (corresponding to 0.59% by weight of sodium methoxide) in order to keep salt deposits as small as possible and the reactor is stirred to keep the precipitated salts in suspension.
A high outlay in terms of apparatus is required for carrying out this known process; in particular, stirring under the high pressure results in virtually insoluble engineering problems. Despite the high engineering outlay, it is not possible to prevent the difficulties associated with the formation of solid deposits in this process either. Trouble-free continuous operation of the process for acceptable periods of time is therefore not possible. For this reason, the production capacity achievable in practice is unsatisfactory and the process as a whole is uneconomical.
DE-C-1046602 discloses a continuous, two-stage process for preparing methyl formate, in the first stage of which methanol is reacted in a jacket-cooled tube reactor to a conversion of from 70 to 75% with CO, which is fed into the tube reactor in substreams at a plurality of points, at from 60 to 140° C. and a pressure of from 50 to 300 bar in turbulent flow in the presence of from 0.5 to 5% by weight of alkali metal alkanolate. The reaction mixture obtained in the first stage is then reacted with an excess of CO in an autoclave until a conversion to methyl formate of about 90%, based on methanol used, has been achieved. in this process, it is difficult to ensure the conditions required in the reactor, namely maintenance of a particular reaction temperature with simultaneous turbulent flow. In addition, industrial implementation of the process requires very difficult, very exact control of the cooling water temperature, the CO pressure and the flow velocity of the reactants. In the second stage of the process, cooling is supposed to be achieved only by introduction of precooled carbon monoxide and/or by subcooling the reaction mixture coming from the first stage. Both measures require a high outlay in terms of apparatus if malfunctions caused by encrustation are to be prevented. Furthermore, it is necessary to use a particularly pure, if necessary prepurified CO in order to avoid precipitate formation and fouling of the cooling surfaces. Although a production capacity of 1674 kg/m3/h has been derived purely mathematically in the working example, this process, too, could not be introduced for continuous production of methyl formate because of the high engineering outlay, the tremendous difficulties in setting and monitoring the process conditions and the remaining intrinsic risk of blockages.
The applicants for this patent have made intensive efforts to overcome difficulties occurring in this process. These efforts have led to the process described in DE-C-1 147 214. Here, at least two substreams of carbon monoxide are fed at different levels into a tower-like reactor, so as to form different reaction zones in the reactor. In the first zone, about 75-85% of the methanol fed in, which contains from 0.12 to 0.3 mol % of alkali metal methoxide, is converted into methyl formate under a CO pressure of from 150 to 200 bar at from 30 to 100° C., and in the lower zones of the reactor the reaction is continued at from 40 to 60° C. to a conversion of about 95%. The salt-like precipitates which are naturally formed in this process are supposed to be prevented from depositing on the plant components by a sudden change in the flow of the various CO substreams carried out at regular or irregular time intervals, with the total flow being kept constant, by ensuring sufficient flow velocities of the reaction mixture and by repeated sudden opening and closing of valves through which flow occurs. However, according to the working example, this process gives a production capacity of only about 440 kg/m3/h in a limited-term individual experiment.
This process variant, too, requires a high outlay for monitoring and for the sudden changes in the CO streams and the valve control to be adapted to the operating values. Despite this, deposits of catalyst and its decomposition products on plant components cannot be avoided over a prolonged operating time, which results in downtimes and a further reduction in the product capacity.
In a further high-pressure process described in DE-A-195 06 555, methanol is reacted at from 50 to 150° C. with CO under a pressure of from 210 to 250 bar in the presence of a relatively low catalyst concentration of from 0.05 to 0.2% by weight of alkali metal methoxide in a reactor or a reactor cascade. Rapid reaction is supposed to be achieved by particularly good dispersion of the carbon monoxide introduced in the reaction mixture, which is supposed to be achieved by blowing it in by means of a jet nozzle. The CO stream can be fed in as substreams and any reactor cascade used can have a temperature profile. The reaction mixture discharged from the reactor contains about 97% by weight of methyl formate and, according to the examples, the production capacity is from 530 to 960 kg/m3/h.
Unreacted methanol is re-used after distillation of the reaction mixture. However, recirculation of the catalyst is not provided for, so that relatively large amounts of solid waste are formed after work-up of the crude product despite the low catalyst concentration. Furthermore, exploitation of the high conversion which is possible in principle involves an appreciable risk of deposition of encrustations in the plant, so that here, too, one has to expect plant downtimes which will drastically reduce the intrinsically good production capacities determined in individual experiments. In addition, the capital costs of the plant are high because of the high process pressures, so that the economics of the process are not fully satisfactory.
The abovementioned high-pressure processes make it possible to achieve a high methanol and CO conversion, but lead to the engineering difficulties and economic disadvantages described, especially high capital costs and salt deposits in the plant components.
A process-engineering alternative to the high-pressure processes is provided by the low-pressure processes which operate at lower CO pressures of from about 10 to 100 bar. These reaction conditions lead to a lower methanol conversion, but generally avoid precipitation of salts. Low-pressure processes are also known in various embodiments.
German patent No. 863 046 discloses a process for the continuous preparation of methyl formate, an improved embodiment of which has been described in German patent No. 880 588. In this process, a solution of sodium alkanolate containing from 1 to 2.5% by weight of sodium (corresponding to from 2.3 to 5.9% by weight of sodium methoxide when using methanol) is reacted with carbon monoxide at from 85 to 90° C. under a pressure of from 10 to 30 bar. In the process of German patent No. 863 046, the alkanolic sodium alkanolate solution and the carbon monoxide are passed once in countercurrent through the reactor, while in the improved process of DE-C-880 588 the reactants are circulated in cocurrent through the reactor. The reaction conditions temperature, flow velocity of the liquid phase and pressure are set so that at least that amount of alkanol required to keep the alkali metal alkanolates used as catalyst in solution remains unconverted. In the improved process, the methyl formate formed during passage through the reactor and excess methanol are discharged from the reactor together with the CO stream in amounts corresponding to their equilibrium vapor pressures. The flow of the CO stream should be set so that the methyl formate is removed as completely as possible from the system. The CO stream leaving the reactor is cooled to condense the entrained compounds methanol and methyl formate and the liquid mixture is discharged from the circuit. The cold carbon monoxide is then once more preheated to the reaction temperature and recirculated to the reactor.
The methanol/methyl formate mixture obtained in this way contains from 38 to 40% by weight of methanol. It is fractionally distilled and the methanol is likewise returned to the circuit. In the improved embodiment of the process as described in DE-C-880 588, a yield of only 3.1 kg of methyl formate is obtained per hour from a reactor having a volume of 770 l, according to the data in the working example. This corresponds to a production capacity of only 4.0 kg/m3/h. A process having a production capacity as low as this is completely out of the question for production on an industrial scale. In addition, the unusually high catalyst concentration is accompanied by considerable disadvantages: during the reaction, there is generally a progressive reduction in the catalyst activity because of the unavoidable formation of alkali metal formate. For this reason, part of the circulating, catalyst-containing methanol always has to be discharged and a corresponding amount of fresh catalyst solution has to be added. The energy consumption required in this process for cooling and reheating the circulating carbon monoxide and for distilling the dilute methyl formate is also not inconsiderable.
German patent 22 43 811 discloses a process for the continuous preparation of methyl formate in which methanol is reacted in countercurrent with gases comprising carbon monoxide at from 50 to 130° C. in the presence of from 0.4 to 1.5% by weight of alkali metal methoxide in a column having flooded, preferably individually cooled trays.
The CO partial pressure should be in the range from 40 to 150 bar and the residence time of the reactants in the column should be from 50 to 1500 seconds. The reaction mixture obtained at the bottom after passing through the column contains from 20 to 70% by weight of methyl formate. It is worked up by distillation.
Apart from the high capital costs of the complicated column construction, a major disadvantage is that only a fraction of the components methanol and carbon mon-oxide fed in is consumed in the process and the high proportion of catalyst is poorly utilized. This leads to additional high costs which are increased further by the energy consumption for preheating the CO and for distillation of all the reaction mixture taken from the bottom of the column. Finally, there are technical problems in disposing of the catalyst- and salt-containing residues from the distillation.
U.S. Pat. No. 4,661,624 describes a process for preparing methyl formate, in which methanol is reacted with CO at from 70 to 130° C. under a pressure of from 5 to 70 bar in the presence of from 1 to 8 mol % (based on the alcohol used, corresponding to from 1.7 to 13.5% by weight of sodium methoxide) of a sodium alkoxide catalyst. The process is controlled so that the conversion is restricted to from 2 to 10% of the alcohol used, as a result of which salt deposits in the reactors are completely avoided but the content of methyl formate in the reaction product is only 1-19% by weight. The reaction mixture discharged is worked up by distillation, which requires an unjustifiably high amount of energy because of the low methyl formate content, even when utilizing the heat of reaction in an integrated heat system. The methanol obtained after the distillation together with the catalyst dissolved therein is returned to the process. However, due to the high concentration of active catalyst required, it is necessary to continually add relatively large amounts of fresh catalyst to the process. In addition, the utilization of the gas comprising carbon monoxide fed in is not satisfactory.
DE-A-43 09 731 relates to a process for preparing methyl formate, in which methanol is partially reacted in a mixing zone with carbon monoxide or gases comprising carbon monoxide at from 60 to 100° C. under a pressure of from 10 to 300 bar (in the working example, 57 bar) in the presence of from 0.4 to 1.5% by weight of alkali metal methoxide. The mixture obtained there is saturated with CO and passed to an after-reaction zone where the reaction is completed without introduction of further starting materials. In this process, too, only a low concentration of methyl formate in the reaction product is obtained, so that the energy balance for the process is unfavorable.
DE-C-27 10 726 discloses a process for preparing methyl formate in which a solution of from 0.2 to 2.5% by weight of an alkali metal methoxide or alkaline earth metal methoxide in methanol is circulated through a reactor in which the liquid phase is present in a lower section and whose upper part contains a gas phase containing from 20 to 100% by volume of carbon monoxide. The reaction is carried out at from 70 to 110° C. and a pressure of from 20 to 110 bar. The methanolic catalyst solution is fed to the reactor using an apparatus (e.g. an impingement plate) which atomizes the liquid jet injected at high velocity in the gas phase, or which draws in gas from the gas phase and injects it as fine bubbles into the liquid phase (e.g. a Venturi tube).
The liquid circuit contains a heat exchanger which allows precise setting of the temperature in the circulating liquid and a powerful pump which maintains the turbulent liquid flow. In operation, fresh methanolic methoxide solution is continuously fed to the reactor and a corresponding amount of the reaction mixture is discharged and has to be worked up by distillation.
According to this document, it is advantageous for thermodynamic reasons to set the feed of starting materials and the discharge in such a way that the reaction mixture contains from about 44 to 65% by weight of methyl formate. If the formation of precipitates is accepted, a reactor output having a methyl formate content of 82% by weight can be obtained. Although such precipitates are supposed to be tolerable according to the document, this is based on only one individual experiment carried out over a limited time. In any case, such precipitates lead to increased abrasion of the high-throughput circulation pump and any impingement elements or Venturi tube used. Isolation of methyl formate from a mixture containing only from 44 to 65% by weight of this substance leads to high energy costs. High operating costs of the process also result from the high catalyst consumption caused by the high catalyst concentration (in 5 of 6 examples, 2.5% by weight) and from the expense of properly disposing of the distillation residue. A further disadvantage of the process is that an increase in the methyl formate content of the reaction product causes a reduction in the production capacity.
Various attempts have also been made to overcome the disadvantages and difficulties associated with the above-described known processes by means of routes other than those followed hitherto.
Thus, prevention of precipitate formation and/or improvements in the production capacity by addition of complexing agents, in particular cyclic polyethers (EP-B-0 048 891), of surface-active solubilizers such as alkali metal perfluoroalkanesulfonates (EP-B0 596 483) or of inert, polar, aprotic solvents (EP-A-0 251 112) have been described. Other attempts comprise the addition of other catalysts such as amidine derivatives (EP-A-0 104 875) or of combinations of amines with ethylene oxide. These processes all have the disadvantage that they introduce additional organic materials into the reaction mixtures and these materials have to be disposed of after the work-up, which eliminates the benefits resulting from any yield improvements achieved. In addition, the purchase price of the proposed additives presents an insuperable obstacle to their industrial use.